Harry Potter and the Black Order

The summer before his Fifth Year, Harry met a white-haired teen with a red left hand and the biggest scar on his face, who destroyed the dementors when the creatures attacked him in Little Whinging. Cue then a massive non-freakout when Harry was never accused of using magic out of school... and his non-introduction to the Order of the Phoenix, as his presence wasn't required nor necessary at Grimmauld Place.
Luckily, Allen Walker was around there as few time as needed to see Harry's treatment at his relatives' hands, and being who he is, he can at least help him escape that shithole... and maybe take him under his wing for the little while until he returns to the Black Order's Headquarters. After all, a wizard's help could be useful when fighting the Holy War, and hopefully Komui can at least determine his health.